1. Field of the Invention
The present invention relates to an oxygen sensor for detecting an oxygen concentration in an exhaust gas discharged from internal combustion engines, and more particularly, to a water-proof structure for the oxygen sensor wherein atmospheric pressure is used as a reference gas.
2. Related Art Statement
Heretofore, it has been known to detect oxygen concentration in the exhaust gas of internal combustion engines, such as automobile engines, etc., by using an oxygen ionic conductive solid electrolyte, according to the principle of oxygen concentration cell, to regulate an air-fuel ratio of the internal combustion engines. Oxygen sensors of this type utilize a tube with a closed end of a zirconia solid electrolyte provided on its inside and outside surfaces respectively with a porous platinum electrode, the inner electrode communicating with atmospheric air to form an electrode having a reference oxygen concentration, while the outer electrode, which functions as a measuring electrode, is exposed to the gas to be measured, i.e., exhaust gas. The communication with the atmosphere is achieved by providing, as an air passage, a clearance between a housing supporting the solid electrolyte tube and a metallic sleeve, or an aperture provided through the metallic sleeve or an electrode terminal for detecting oxygen concentration.
However, such conventional oxygen sensors generally have a drawback, when used in internal combustion engines, that water, brine, mud, etc. enter from the air passage, causing the solid electrolyte, under a high temperature condition, to break or the electrode can become corroded.
In order to solve the above-mentioned problem, an air passage provided on a metallic sleeve has been covered by a filter, as proposed in U.S. Pat. No. 4,116,797.
In this oxygen sensor, however, since the filter is only inserted into the air passage of the metallic sleeve, upon exposure of the oxygen sensor to a high temperature ambience in an internal combustion engine, the filter is prone to be detached due to a difference in thermal expansion between the filter and the metal, and in particular, the above problem occurs in the case where a heater member for heating the solid electrolyte is provided inside the solid electrolyte tube for the purpose of enabling the oxygen sensor to accurately detect oxygen concentration when internal combustion engines are first started.
Further, in such an oxygen sensor provided with a heater member inside, a lead wire for supplying power to the heater member, in addition to the electrode terminal for detecting oxygen concentration, must be inserted through the rubber grommet so that it is difficult to provide a filter in a limited, narrow area of the rubber grommet.